In recent years, following the development of a highly-integrated semiconductor product, such as a semiconductor memory and a VLSI (Very Large Scale Integrated circuit), there arises a demand for a fine pattern exceeding a transfer limit in photolithography. In order to enable such a fine pattern to be transferred, proposal has been made of lithography using extreme ultraviolet light (hereinafter abbreviated to EUV light) having a shorter wavelength or the like. It is noted here that the EUV light means light of a wavelength band within a soft X-ray region or a vacuum ultraviolet region, specifically, light having a wavelength of about 0.2-100 nm.
In the meanwhile, proposal has been made of a reflective mask used as an exposure mask within a short-wavelength region including the EUV light or X-ray. The reflective mask has a basic structure comprising a Si or quartz substrate, a reflective layer formed on the substrate to reflect the EUV light or the X-ray, and an absorber pattern formed on the reflective layer to absorb the EUV light or the X-ray. Generally, the reflective layer comprises a multilayer film including thin films of at least two kinds of substances alternately laminated. In a direction inclined at several degrees (typically, 2 to 5 degrees) from a perpendicular direction to the mask, exposure light is incident to the mask. The exposure light is absorbed in an area where the absorber pattern is present. In a remaining area, the exposure light is reflected by the reflective layer. Therefore, a reflected image corresponding to the absorber pattern is formed. By reduction projection of the reflected image through an appropriate optical system onto a silicon wafer, transfer is carried out.
Japanese Patent Application Publication (JP-A) Nos. H7-333829 and H8-213303 disclose a structure including an intermediate layer formed between the reflective layer and the absorber, in addition to the above-mentioned basic structure of the reflective mask. Thus, the intermediate layer is formed in order to protect the reflective layer during patterning of the absorber, in particular, during etching so that the reflective layer as an underlying layer is not damaged by etching.
Now, referring to FIG. 1, description will be made of a method of producing a reflective mask used in the lithography using the EUV light (for example, the EUV light having a wavelength of about 13.4 nm within the soft X-ray region). FIG. 1 includes schematic sectional views sequentially showing a production process of an existing reflective mask.
A mask blank 101 is prepared by successively depositing, on a substrate 11 of quartz or the like, a laminate film 12 as a reflective layer for the EUV light (hereinafter called an EUV reflective layer), a buffer layer 13 (corresponding to the above-mentioned intermediate layer) formed on the laminate film for the purpose of protecting the EUV reflective layer in an absorber pattern forming step, an absorber layer 14 formed on the buffer layer to absorb the EUV light (hereinafter called an EUV absorber layer) (see FIG. 1(a)).
Next, the EUV absorber layer 14 as an absorber for the EUV light is processed to form an EUV absorber pattern having a predetermined pattern (see FIG. 1(b)).
Then, inspection is carried out to confirm whether or not the EUV absorber pattern is formed exactly as designed. For example, it is assumed that, as a result of the pattern inspection, detection is made of occurrence of a pinhole defect (a defect that the absorber layer is removed at a position where it should not be etched off, may be called a white (clear) defect) 21 resulting from adhesion of foreign matters to a resist layer during pattern formation and an underetching defect (a position where the absorber layer is not sufficiently removed due to underetching, may be called a black (opaque) defect) 22, as shown in FIG. 1(b). In this event, the pinhole defect 21 is repaired by depositing a carbon film 23 in a pinhole using focused ion beam (FIB) assisted deposition. The underetching defect 22 is repaired by removing a residual part 22a using FIB gas assisted etching to obtain a removed part 25 where the absorber layer 14 is removed. By irradiation energy during repair, a damaged part 24 (a part 24a removed by FIB and a part 24b penetrated by FIB ions) is present on a surface of the buffer layer 13 (see FIG. 1(c)).
Thereafter, by removing a part of the buffer layer 13 corresponding to the removed part 25 where the EUV absorber layer 14 is removed, a pattern 26 is formed so that the reflective mask for the EUV light is produced (see FIG. 1(d)).
When the reflective mask is exposed by EUV light 31, the light is absorbed in an area where the absorber pattern is present. In a remaining area (where the absorber layer 14 and the buffer layer 13 are removed), the EUV light 31 is reflected by the reflective layer 12 which is exposed (see FIG. 1(e)). Thus, the reflective mask can be used as a mask for the lithography using the EUV light.
As described above, in the above-mentioned mask production process, inspection is carried out, after the pattern is formed on the EUV absorber layer 14, to confirm whether or not the EUV absorber pattern is formed exactly as designed. The inspection of the mask pattern is carried out by the use of an inspecting apparatus using light having a wavelength of, for example, about 257 nm (generally, deep ultraviolet light having a wavelength of 190-260 nm). Specifically, by irradiating the mask with the light of about 257 nm, a pattern of a reflected image is produced to be subjected to the inspection. The inspection of the mask pattern is carried out after completion of the pattern forming step (step in FIG. 1(b)) for the EUV absorber layer 14 on a surface as described above. Based on the result of the inspection, the pattern is repaired if necessary. Specifically, the inspection is carried out by a difference in reflectivity between the surface of the buffer layer 13 exposed after the absorber on the surface is removed by patterning and the surface of the absorber as the remaining pattern when the light used in the inspection (hereinafter will be referred to as inspection light) is irradiated onto the mask. Therefore, if the difference in reflectivity for the wavelength of the inspection light between the surface of the buffer layer and the surface of the absorber layer is small, a contrast upon the inspection is insufficient so that defect inspection can not accurately be carried out.
Typically, in case of the existing reflective mask, the EUV absorber on the surface is formed by a tantalum film or a tantalum nitride film and the buffer layer is formed by a SiO2 film. For the inspection light having a wavelength of 257 nm, the difference between the reflectivity of the surface of the absorber and the reflectivity on the surface of the buffer layer is small so that the contrast upon the inspection is insufficient. As a result, a pattern defect can not sufficiently be detected during the inspection of the mask and an accurate defect test can not be carried out.
On the other hand, by inspection with an electron microscope using an electron beam, an EUV absorber film is damaged by the electron beam irradiated thereto. Therefore, practical use is difficult.
Proposal is also made of a method of using light of about 13.4 nm as an EUV light wavelength mentioned above to inspect a mask pattern. However, in order to equip the inspection apparatus with an EUV light source, an extremely high facility cost is required. Further, as compared with an existing inspection apparatus using an ultraviolet wavelength, a pattern inspecting step is increased in scale and complicated because a structure of holding a whole of an optical system in vacuum is required in order to avoid absorption in atmospheric air. In addition, a throughput is reduced due to a time required for evacuation into vacuum.
Referring to FIG. 1, description will be made of a specific example where the reflective layer 12 is a multilayer reflective film. Specifically, the multilayer reflective film comprising thin films made of substances different in refractive index and alternately laminated is generally used as the reflective layer 12. For example, as the multilayer reflective film for light having a wavelength of about 13 nm, a multilayer film comprising Si and Mo alternately laminated in about 40 periods is known.
In the specific example, it is assumed that the buffer layer 13 comprises a SiO2 film or a Cr film and the absorber layer 14 is made of Ta or a Ta alloy.
After the step in FIG. 1(d) (the step of removing a predetermined part of the buffer layer 13 on the reflective layer 12 to form the pattern 26), inspection for final confirmation is carried out to confirm whether or not the absorber pattern is formed in exact conformity with a specification. The final inspection of the pattern is also carried out by observing the contrast in reflection of the inspection light on the surface of the mask using deep ultraviolet light as the inspection light, in the manner similar to the above-mentioned first inspection after completion of the pattern forming step (the step in FIG. 1(b)) for the absorber layer 14.
Specifically, in the first inspection, the inspection is carried out by observing the contrast in reflection of the inspection light between the surface of the buffer layer 13 exposed in the area where the absorber layer 14 is removed and the surface of the absorber layer 14 in the area where the absorber layer 14 remains. On the other hand, in the inspection for final confirmation, the inspection is carried out by the contrast in reflection of the inspection light between the surface of the multilayer reflective film 12 exposed in the area where the buffer layer 13 is removed and the surface of the absorber layer 14 in the area where the absorber layer 14 remains.
Therefore, if the difference in reflectivity between the surface of the buffer layer 13 and the surface of the absorber layer 14 for the wavelength of the inspection light is small, the contrast during the first inspection is inferior so that the first inspection can not accurately be carried out. If the difference in reflectivity between the surface of the multilayer reflective film and the surface of the absorber layer for the wavelength of the inspection light is small, the contrast during the final inspection is inferior so that the final inspection can not accurately be carried out.
For example, in case where the deep ultraviolet light having a wavelength of 257 nm is used as the inspection light, the reflectivity of Ta or the Ta alloy used as the absorber layer for the EUV light is as relatively high as about 35%. On the other hand, the reflectivity of the buffer layer is about 40% in case of SiO2 and about 50% in case of Cr. Therefore, the difference in reflectivity is small so that a sufficient contrast can not be obtained in the pattern inspection. A Mo/Si periodic multilayer film generally used for the exposure light having a wavelength of about 13 nm has a reflectivity of about 60% for far ultraviolet light. In this case, it is also difficult to achieve a contrast sufficient to obtain an accurate result in the inspection for final confirmation.
On the other hand, it is possible to decrease the reflectivity for the inspection light by roughening the surface of the absorber layer. In this case, however, edge roughness after pattern formation is increased so that the dimensional accuracy of the mask is degraded.
In order to decrease the reflectivity, it is effective to add nitrogen. However, tantalum nitride (TaN) containing Ta with nitrogen added thereto is a crystalline substance. In particular, if a metal film is used as the buffer layer, a TaN film formed thereon has a granular structure. In this case also, the edge roughness after pattern formation is increased and the dimensional accuracy of the mask is degraded.